I don't know its use but its a more general file maybe obsolete (very old stuff in it). Look at the date its from 2001/09/16 and I re-installed gentoo just a few days ago.
the information (for emerge) is in /var/db/pkg. emerge gentoolkit has some nice tools where you can do some queries. qpkg for example gives the list of installed packages, qpkg -f /usr/.../<file> gives the package which owns the file, well you get my drift.

That's right. The current-packages file is not really used for anything. I don't really know that it was ever used, but it was probably just the output of a find command. The best way to get a list of all packages is